1. Field of the Invention
The present invention relates to an image reading apparatus, and in particular to an image reading apparatus capable of compensating unevenness in the output in image reading.
2. Related Background Art
There are already known various apparatus capable of image reading, such as facsimiles or copying machines, and the image reading in such apparatus is mostly conducted by reflected light obtained by illuminating an original document with a light source. Such apparatus are usually provided with a device for correcting shading distortion in the obtained image signals or maintaining the obtained image signals at a constant gain, thereby attaining a constant reading condition, since light sources, such as flourescent lamp are usually unable to provide a constant intensity over the entire length thereof and show a change in light intensity over time, and also the image reading element is associated with fluctuation and deterioration over time.
FIG. 1 shows an image reading apparatus with conventional correction for shading distortion.
In FIG. 1, the reflected light from an original is focused, through an optical system such as a lens or a prism, onto a CCD sensor 101, functioning as a photoelectric converting element.
A white reference plate 100 is provided in a predetermined position in the image reading system, and the light reflected from said white reference plate is introduced into the CCD sensor 101 in order to obtain data on the shading distortion.
The output signal from the CCD sensor 101 is released through a variable gain amplifier 104. Also the CCD sensor 101 is connected to an A/D converter 102 to store the output of the CCD sensor 101 in a random access memory (RAM) 135 in the form of digital data of plural bits representing output level. The output of said RAM 135 is converted in a decoder 103 to control the gain of said variable gain amplifier 104 and is supplied to a control input terminal thereof.
In the above-explained structure, the white reference plate 100 is scanned by the CCD sensor 101 prior to the image reading, and the output thereof is converted into digital data by the A/D converter 102 and stored in the RAM 135. For example, a deficiency in the light intensity may occur in an end area of the reference plate 100 due to uneven distribution of the light intensity of the light source, and corresponding data are stored in the RAM 135.
Subsequently, an original to be read is scanned by the CCD sensor 101, and, at the same time data are read from the RAM 135 in synchronization with the output of image signals from the CCD sensor 101. The data read from the RAM 135 are supplied, through the decoder 103, to the amplifier 104 to regulate the gain thereof. In this manner it is rendered possible to correct the shading distortion for example in end portions, and to obtain uniform image data over the entire area of the original.
In such conventional structure, however, it has been difficult to reduce the cost since a random access memory of a large capacity is required for storing image data converted into digital data by the A/D converter. Particularly in cases where high-speed processing is required, there will be required an A/D converter capable of functioning at a high speed, thus complicating the structure and elevating the cost.